Mobile communication systems are a ubiquitous part of modern life. Ongoing trends of mobile communication devices (e.g., radiotelephones) include decreasing size and increases in features and capabilities. Both trends are driven by the shrinking size and higher integration of electronics, particularly digital electronics. CMOS electronics, in particular, benefit from greater area utilization, lower power consumption, higher speed, and lower cost as CMOS technology migrates to ever-smaller feature size and higher integration.
Radio frequency (RF) circuits, on the other hand, which remain largely analog, have experienced no such dramatic improvements. Analog circuits thus consume an ever-increasing proportion of the circuit area and power of mobile communication devices, as digital circuits continue to shrink.
For RF transmitter design, the traditional quadrature transmitter architecture depicted in FIG. 20 remains the dominant design. The transmitter 100 includes signal processor 102 generating in-phase (I) and quadrature (Q) components of signals to be transmitted. Digital-to-Analog Converter 104 converts the digital signals to analog signals, using a reconstruction clock fs provided by a clock generator circuit 106. A Low Pass Filter 108 acts like a reconstruction filter and limits the analog signal bandwidth. The signals are modulated by a quadrature modulator 110, using quadrature clocks at the carrier frequency ftx, provided by a clock driver circuit 112. A Variable Gain Amplifier 114 amplifies the modulated signal, and a Power Amplifier 116 boosts the signal to a power level sufficient at the output of the transmitter connected to an antenna 118.
The non-linearity in the components along the transmitter 100 chain can create harmonic distortions and inter-modulation products, which are unwanted frequency components leading to spurious emissions and interference to neighbor receivers or even its own receiver. To avoid this, the linearity requirements for the LPFs 108, quadrature modulator 110, and VGA 114 are very high, increasing the difficulty in design of these components, as well as their cost. On the other hand, high linearity normally implies high power consumption, as these analog components are usually operating in class A.
The traditional quadrature modulated transmitter 100 is deficient in several respects. Active and passive components, such as filter capacitors and large transistors for minimizing flicker noise, occupy a large silicon area, and the area will not shrink significantly with CMOS technology migration. Additionally, the analog design is sensitive to process variations, temperature and supply voltage changes, and the like, hence it is more sensitive to its environment and more difficult to design, e.g., in dealing with different process corners. Device matching is also a problem for deep submicron CMOS. Finally, power consumption is higher because better linearity and lower noise are required, which lead to high power.
To relax the design effort and reduce area and power consumption, an integrated digital quadrature modulator transmitter 120, depicted in FIG. 21, has been proposed by He, in the paper, “A 45 nm Low-power SAW-less WCDMA transmit Modulator Using Direct Quadrature Voltage Modulation,” presented at the ISSCC, February 2009 in San Francisco, USA. The transmitter 120 features a digital quadrature modulator 122 that merges many components in a single digital block. VGA 124 sets the transmitter chain gain digitally by a multiplication operation. The original sample ratio of the baseband signal may be insufficient for digital-to-RF conversion stage DAC/QMOD, i.e., the quadrature modulator 130 which creates modulation spurious spectrum around the carrier clock frequency, thus the quadrature modulator 122 oversamples the signal, and uses interpolation circuits 126 to achieve the necessary bandwidth. The low pass filters 128 remove high frequency harmonics, and the combined DAC and quadrature modulator 130 generate an RF, quadrature modulated signal. Clock generator 106 and clock driver 112 provide carrier clock signals, which are modulated by the digital baseband signals and converted into modulated RF signals. Since the digital baseband signals have lower distortion than their analog counterparts, the linearity is improved. Additionally, area consumption may also be less than the analog components, due to the removal of large capacitors.
While it may address several shortcomings of the traditional design 100 of FIG. 20, the integrated digital quadrature modulator transmitter 120 of FIG. 21 also has several shortcomings. Although area efficiency is better than the transmitter 100, the power amplifier 116 remains as a separate circuit. The necessity of a separate modulator 122 and power amplifier 116, including the pads and interconnection, represents redundant area. Also, the power consumption is higher at the modulator 122 output, as it is normally required to drive 50 Ohm impedance at the output. The power efficiency in the modulator 122 and the power amplifier 116 is lower, due to the fact that they are normally operating linearly in class A, where the power consumption is constant. Thus, the design 120 has very low power efficiency at low output power levels. Furthermore, non-linear distortion in the power amplifier 116 is difficult to compensate, which creates interference at other radio frequencies. Finally, the system integration is not optimized, increasing the cost of the transmitter.
Regardless of the configuration or level of integration of the transmitter 100, 120 components, quadrature modulation has some basic deficiencies. In quadrature modulation, orthogonal signal components I and Q are used in the modulation which is very simple and straight-forward, as the baseband processor normally creates I and Q signals. Additionally, the I and Q channels may be balanced to reduce interference and disturbance in receiver band. However, a remaining issue in the power amplifier 116 is the efficiency dropping when I and Q channel outputs are combined together. With a Wilkinson power combiner, the efficiency drops 3 dB, and with an inductive load power combiner, when both I and Q conduct, the drain efficiency also drops. FIG. 22 depicts the results of a simulation showing that the efficiency drops most when amplitude of I equals that of Q, i.e., at 45 degree direction, when the I and Q channels most load each other.
A known alternative to quadrature modulation is polar modulation. In polar modulation, the amplitude signal m ism=√{square root over (I2+Q2)},where m≧0 and the phase, or angle, signals is
  θ  =      arctan    ⁡          (              Q        I            )      where 0≦θ≦2π. The sinusoid functions at the angle θ are
            sin      ⁡              (        θ        )              =          Q      m                  cos      ⁡              (        θ        )              =          I      m      
Polar modulation does not suffer the same drop in efficiency near 45 degrees, as the amplitude and phase modulation paths are different, and do not load each other. However, the difference in the two paths introduces difficulties in matching them, which leads to different group delays. This introduces distortion, e.g., as measured by the error vector magnitude (EVM), which is the ratio of distortion to signal amplitude. Additionally, there is a large bandwidth expansion (e.g., on the order of five to nine times) in the conversion from Cartesian (I, Q) to polar (amplitude, phase) coordinates. The conversion is non-linear, with sharp edges when the signal is close to zero amplitude that require large bandwidth to reduce the distortion. This makes polar modulation unsuited for wideband applications, such as WCDMA.
A modulation technique that exhibited the consistent group delay and low bandwidth of quadrature modulation, with the consistent efficiency of polar modulation—and additionally would allow for an all-digital implementation, to take advantage of the speed and power benefits of CMOS technology evolution—would represent a significant advance in the state of the art of communication signal transmission.